With the rapid development of power electronics, Printed Circuit Boards (PCBs) are widely applied in product designs and developments. In order to meet the requirements such as high performance and high reliability of products, the layout and wiring of PCBs becomes a focus in the field.
A multilayer PCB includes a plurality of conductor layers and a plurality of insulation layers disposed between adjacent conductor layers. In the layout of a multilayer PCB (two or more layers), vias are generally used for realizing electrical connections between two or more conductor layers. If there are more than three conductor layers in a PCB, the inner conductor layer have to be electrically connected with the electronic devices on the outer conductor layers by means of vias, such as through holes, blind vias or buried vias.
The arrangements of vias in a PCB will be described below. There are generally the following types of arrangements.
1. One-to-One Type
The one-to-one type arrangement generally refers to that one or more conductor layers in a PCB are connected with another one or more conductor layers using vias, and the number of the one or more conductor layers is the same as the number of the another one or more conductor layers.
FIG. 1 is a schematic diagram showing a conventional one-to-one type via arrangement. As shown in FIG. 1, a conductor layer 81 in a PCB is connected with another conductor layer 82 using a plurality of vias 83. In order to realize reliable connections between the conductor layers 81 and 82, a plurality of rows of vias 83 may be used and the numbers of the vias 83 in respective rows are the same. In FIG. 1, a total of six rows of vias each of which includes five vias are used for the connections between the conductor layers 81 and 82. In addition, for the convenience in description, insulation layers (for example, base materials formed by FR4) disposed between the conductor layers in the PCB are omitted in this figure and thereby related descriptions are note elaborated here.
2. One-to-Many Type Arrangement
The one-to-many type arrangement generally refers to that one or more conductor layers in a PCB are connected with a plurality of conductor layers using vias, and the number of the one or more conductor layers is different from the number of the plurality of conductor layers.
FIG. 2 is a schematic diagram showing a conventional one-to-many via arrangement. As shown in FIG. 2, a conductor layer 91 in a PCB is connected with other three conductor layers (i.e., conductor layers 92, 93 and 94) using vias 95. Similarly, in order to realize reliable connections between the conductor layer 91 and the other three conductor layers, total of six rows of vias each of which includes five vias are used for the connections between the conductor layers 91 and the other three conductor layers.
As can be seen from the above connections using vias, if it is desired to realize reliable connections between conductor layers, the prevailed method in conventional technologies is to increase the number of vias. However, in most cases, due to the limitation of the area of a PCB, the number of vias cannot be increased too much.
Further, due to limitations of existing manufacturing steps, walls of the vias are relatively thin and therefore the impedance of the vias is relatively large. Also, there is an overlapping region between the conductor layers using vias, and the current flowing through the circuit will converge at an edge of an overlapping region. Consequently, the vias in outer rows will withstand the converged current and the current density of the current flowing through the vias in outer rows will become large. Besides, when vias using one-to-many type arrangement, the current density of the current flowing through the vias in a smaller number of the conductor layers will become even larger, and the loss of these vias will be increased rapidly, thereby these vias in outer rows will be subject to high temperatures and may seriously influence the lifetimes of the PCBs.